This invention relates to an image forming apparatus for detecting toner concentration and stably controlling the toner concentration, such as a copier or a laser printer, and a control method of the image forming apparatus.
An image forming apparatus, for example, a color copier has a configuration as shown in FIG. 30 and comprises a photoelectric sensor 30 for detecting a toner image consisting of percentage of image coverage prepared on a photoreceptor 10 to stably control a toner concentration transferred to recording paper.
The photoelectric sensor 30 used is a photoelectric sensor for detecting specular reflection light on the surface of the photoreceptor 10 as shown in FIG. 31, a sectional view, or a photoelectric sensor for detecting diffuse reflection light (not shown).
FIG. 32 shows the relationship between the concentration of a toner patch image (test toner image) and sensor output voltage when the photoelectric sensor for detecting specular reflection light is used. As shown here, to use the photoelectric sensor for detecting specular reflection light, when the toner patch concentration is high regardless of black toner or color toner, the sensor output voltage lowers. However, for the color toner, if the toner patch concentration becomes high exceeding a saturation point, the sensor output voltage rises, because diffuse reflection light increases and enters the photoelectric sensor.
Normally, honing treatment is applied to the surface of a photoreceptor to avoid interference fringes of laser light. As shown in FIG. 33, when the photoelectric sensor gets reflected light on the photoreceptor surface, the sensor output voltage is a state in which fine reflection unevenness is largely undulated. That is, the sensor output voltage becomes a state in which fine reflection unevenness caused by the surface state of the honing treatment is undulated by large reflection unevenness caused by vibration at the rotation time of the photoreceptor or the photoreceptor surface working effect.
With a color copier, sensor output voltage fluctuation of the photoelectric sensor caused by the large undulated reflection unevenness of the two types of reflection unevenness causes a problem in stably controlling the toner concentration transferred to recording paper.
The Unexamined Japanese Patent Application Publication No. Hei 4-146459 discloses an art of measuring the sensor output voltages of photoreceptor background, a test toner image at saturation level, and a halftone test toner image by a photoelectric sensor for detecting diffuse reflection light, calculating a developing bias voltage from the measurement values, and feeding back the calculated developing bias voltage to a developing bias power supply unit.
In this art, the measurement timings of the reflected light quantity on the photoreceptor background, the reflected light quantity on the test toner image at the saturation level, and the reflected light quantity on the halftone test toner image by the photoelectric sensor are matched with the rotation cycle of the photoreceptor to avoid the sensor output fluctuation effect of the photoelectric sensor caused by reflection unevenness on the photoreceptor caused by a roundness error, an eccentricity error, or the like on manufacturing the photoreceptor.
The difference between the photoelectric sensor for detecting specular reflection light and the photoelectric sensor for detecting diffuse reflection light is that the sensor output voltage of the latter photoelectric sensor rises when the toner concentration of black toner or color toner is high as the phase of the sensor output voltage responding to the toner concentration is inverted.
However, the reflected light quantity from a toner image is affected by reflection unevenness on the background of a photoreceptor depending on the concentration of the toner image. Considering how the light quantity is affected by reflection unevenness, as shown in FIG. 34, the sensor output voltage is truly affected by reflection unevenness on the background of a photoreceptor at 20% toner patch concentration and has undulation similar to that caused by reflection unevenness on the background, but is little affected by reflection unevenness on the background of the photoreceptor at 60% toner patch concentration and thus little changes.
In the art disclosed in the Unexamined Japanese Patent Application Publication No. Hei 4-146459, the sensor output fluctuation effect of the photoelectric sensor caused by reflection unevenness on the photoreceptor caused by a roundness error, an eccentricity error, or the like on manufacturing the photoreceptor is avoided, but the reflected light quantity is affected by reflection unevenness on the background of the photoreceptor depending on the toner patch concentration. Therefore, an error occurs between the measurement value of the photoelectric sensor when the toner patch concentration is low and the effect of reflection unevenness on the background is received and that when the toner patch concentration is high and the effect of reflection unevenness on the background is not received.